robin_hood.h

//                 ______  _____                 ______                _________
//  ______________ ___  /_ ___(_)_______         ___  /_ ______ ______ ______  /
//  __  ___/_  __ \__  __ \__  / __  __ \        __  __ \_  __ \_  __ \_  __  /
//  _  /    / /_/ /_  /_/ /_  /  _  / / /        _  / / // /_/ // /_/ // /_/ /
//  /_/     \____/ /_.___/ /_/   /_/ /_/ ________/_/ /_/ \____/ \____/ \__,_/
//                                      _/_____/
//
// Fast & memory efficient hashtable based on robin hood hashing for C++11/14/17/20
// https://github.com/martinus/robin-hood-hashing
//
// Licensed under the MIT License <http://opensource.org/licenses/MIT>.
// SPDX-License-Identifier: MIT
// Copyright (c) 2018-2021 Martin Ankerl <http://martin.ankerl.com>
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software";, to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
 
#ifndef ROBIN_HOOD_H_INCLUDED
#define ROBIN_HOOD_H_INCLUDED
 
// see https://semver.org/
#define ROBIN_HOOD_VERSION_MAJOR 3 // for incompatible API changes
#define ROBIN_HOOD_VERSION_MINOR 11 // for adding functionality in a backwards-compatible manner
#define ROBIN_HOOD_VERSION_PATCH 5 // for backwards-compatible bug fixes
 
#include "gaia/config/config_core.h"
 
#include <cstdlib>
#include <cstring>
#include <initializer_list>
#include <new>
#include <tuple>
#include <type_traits>
#include <utility>
 
#include "gaia/core/hashing_policy.h"
#include "gaia/core/iterator.h"
#include "gaia/core/utility.h"
#include "gaia/mem/mem_alloc.h"
#include "gaia/mem/mem_utils.h"
#include "gaia/ser/ser_ct.h"
#include "gaia/util/logging.h"
 
// #define ROBIN_HOOD_STD_SMARTPOINTERS
#if defined(ROBIN_HOOD_STD_SMARTPOINTERS)
  #include <memory>
#endif
 
// #define ROBIN_HOOD_LOG_ENABLED
#ifdef ROBIN_HOOD_LOG_ENABLED
  #define ROBIN_HOOD_LOG(x, ...) GAIA_LOG_D("L:%s@%d: " x, __FUNCTION__, __LINE__, ##__VA_ARGS__)
#else
  #define ROBIN_HOOD_LOG(x, ...)
#endif
 
// #define ROBIN_HOOD_TRACE_ENABLED
#ifdef ROBIN_HOOD_TRACE_ENABLED
  #define ROBIN_HOOD_TRACE(x, ...) GAIA_LOG_D("T:%s@%d: " x, __FUNCTION__, __LINE__, ##__VA_ARGS__)
#else
  #define ROBIN_HOOD_TRACE(x, ...)
#endif
 
// all non-argument macros should use this facility. See
// https://www.fluentcpp.com/2019/05/28/better-macros-better-flags/
#define ROBIN_HOOD(x) ROBIN_HOOD_PRIVATE_DEFINITION_##x()
 
// mark unused members with this macro
#define ROBIN_HOOD_UNUSED(identifier)
 
// bitness
#if SIZE_MAX == UINT32_MAX
  #define ROBIN_HOOD_PRIVATE_DEFINITION_BITNESS() 32
#elif SIZE_MAX == UINT64_MAX
  #define ROBIN_HOOD_PRIVATE_DEFINITION_BITNESS() 64
#else
  #error Unsupported bitness
#endif
 
// exceptions
#if !defined(__cpp_exceptions) && !defined(__EXCEPTIONS) && !defined(_CPPUNWIND)
  #define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_EXCEPTIONS() 0
  #define ROBIN_HOOD_STD_OUT_OF_RANGE void
#else
  #include <stdexcept>
  #define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_EXCEPTIONS() 1
  #define ROBIN_HOOD_STD_OUT_OF_RANGE std::out_of_range
#endif
 
// count leading/trailing bits
#if !defined(ROBIN_HOOD_DISABLE_INTRINSICS)
  #if ROBIN_HOOD_PRIVATE_DEFINITION_BITNESS() == 32
    #define ROBIN_HOOD_COUNT_TRAILING_ZEROES(x) GAIA_CLZ(x)
    #define ROBIN_HOOD_COUNT_LEADING_ZEROES(x) GAIA_CTZ(x)
  #else
    #define ROBIN_HOOD_COUNT_TRAILING_ZEROES(x) GAIA_CLZ64(x)
    #define ROBIN_HOOD_COUNT_LEADING_ZEROES(x) GAIA_CTZ64(x)
  #endif
#endif
 
// fallthrough
#ifndef __has_cpp_attribute // For backwards compatibility
  #define __has_cpp_attribute(x) 0
#endif
#if __has_cpp_attribute(fallthrough)
  #define ROBIN_HOOD_PRIVATE_DEFINITION_FALLTHROUGH() [[fallthrough]]
#else
  #define ROBIN_HOOD_PRIVATE_DEFINITION_FALLTHROUGH()
#endif
 
// detect if native wchar_t type is availiable in MSVC
#ifdef _MSC_VER
  #ifdef _NATIVE_WCHAR_T_DEFINED
    #define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 1
  #else
    #define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 0
  #endif
#else
  #define ROBIN_HOOD_PRIVATE_DEFINITION_HAS_NATIVE_WCHART() 1
#endif
 
// detect if MSVC supports the pair(std::piecewise_construct_t,...) constructor being constexpr
#ifdef _MSC_VER
  #if _MSC_VER <= 1900
    #define ROBIN_HOOD_PRIVATE_DEFINITION_BROKEN_CONSTEXPR() 1
  #else
    #define ROBIN_HOOD_PRIVATE_DEFINITION_BROKEN_CONSTEXPR() 0
  #endif
#else
  #define ROBIN_HOOD_PRIVATE_DEFINITION_BROKEN_CONSTEXPR() 0
#endif
 
// workaround missing "is_trivially_copyable" in g++ < 5.0
// See https://stackoverflow.com/a/31798726/48181
#if GAIA_COMPILER_GCC && __GNUC__ < 5
  #define ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(...) __has_trivial_copy(__VA_ARGS__)
#else
  #define ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(...) std::is_trivially_copyable<__VA_ARGS__>::value
#endif
 
namespace robin_hood {
 
  namespace detail {
 
// make sure we static_cast to the correct type for hash_int
#if ROBIN_HOOD(BITNESS) == 64
    using SizeT = uint64_t;
#else
    using SizeT = uint32_t;
#endif
 
    template <typename T>
    T rotr(T x, unsigned k) {
      return (x >> k) | (x << (8U * sizeof(T) - k));
    }
 
    // This cast gets rid of warnings like "cast from 'uint8_t*' {aka 'unsigned char*'} to
    // 'uint64_t*' {aka 'long unsigned int*'} increases required alignment of target type". Use with
    // care!
    template <typename T>
    inline T reinterpret_cast_no_cast_align_warning(void* ptr) noexcept {
      return reinterpret_cast<T>(ptr);
    }
 
    template <typename T>
    inline T reinterpret_cast_no_cast_align_warning(void const* ptr) noexcept {
      return reinterpret_cast<T>(ptr);
    }
 
    template <typename, typename = void>
    struct has_hash: public std::false_type {};
 
    template <typename T>
    struct has_hash<T, std::void_t<decltype(T::hash)>>: public std::true_type {};
 
    // make sure this is not inlined as it is slow and dramatically enlarges code, thus making other
    // inlinings more difficult. Throws are also generally the slow path.
    template <typename E, typename... Args>
    [[noreturn]] GAIA_NOINLINE
#if ROBIN_HOOD(HAS_EXCEPTIONS)
        void doThrow(Args&&... args) {
      // NOLINTNEXTLINE(cppcoreguidelines-pro-bounds-array-to-pointer-decay)
      throw E(GAIA_FWD(args)...);
    }
#else
        void doThrow(Args&&... ROBIN_HOOD_UNUSED(args) /*unused*/) {
      abort();
    }
#endif
 
    template <typename E, typename T, typename... Args>
    T* assertNotNull(T* t, Args&&... args) {
      if GAIA_UNLIKELY (nullptr == t) {
        doThrow<E>(GAIA_FWD(args)...);
      }
      return t;
    }
 
    template <typename T>
    inline T unaligned_load(void const* ptr) noexcept {
      // using memcpy so we don't get into unaligned load problems.
      // compiler should optimize this very well anyways.
      T t;
      memcpy(&t, ptr, sizeof(T));
      return t;
    }
 
    // Allocates bulks of memory for objects of type T. This deallocates the memory in the destructor,
    // and keeps a linked list of the allocated memory around. Overhead per allocation is the size of a
    // pointer.
    template <typename T, size_t MinNumAllocs = 4, size_t MaxNumAllocs = 256>
    class BulkPoolAllocator {
    public:
      BulkPoolAllocator() noexcept = default;
 
      // does not copy anything, just creates a new allocator.
      BulkPoolAllocator(const BulkPoolAllocator& ROBIN_HOOD_UNUSED(o) /*unused*/) noexcept:
          mHead(nullptr), mListForFree(nullptr) {}
 
      BulkPoolAllocator(BulkPoolAllocator&& o) noexcept: mHead(o.mHead), mListForFree(o.mListForFree) {
        o.mListForFree = nullptr;
        o.mHead = nullptr;
      }
 
      BulkPoolAllocator& operator=(BulkPoolAllocator&& o) noexcept {
        reset();
        mHead = o.mHead;
        mListForFree = o.mListForFree;
        o.mListForFree = nullptr;
        o.mHead = nullptr;
        return *this;
      }
 
      BulkPoolAllocator&
      // NOLINTNEXTLINE(bugprone-unhandled-self-assignment,cert-oop54-cpp)
      operator=(const BulkPoolAllocator& ROBIN_HOOD_UNUSED(o) /*unused*/) noexcept {
        // does not do anything
        return *this;
      }
 
      ~BulkPoolAllocator() noexcept {
        reset();
      }
 
      // Deallocates all allocated memory.
      void reset() noexcept {
        while (mListForFree) {
          T* tmp = *mListForFree;
          ROBIN_HOOD_LOG("gaia::mem::mem_free");
          gaia::mem::mem_free(mListForFree);
          mListForFree = reinterpret_cast_no_cast_align_warning<T**>(tmp);
        }
        mHead = nullptr;
      }
 
      // allocates, but does NOT initialize. Use in-place new constructor, e.g.
      //   T* obj = pool.allocate();
      //   ::new (static_cast<void*>(obj)) T();
      T* allocate() {
        T* tmp = mHead;
        if (tmp == nullptr) {
          tmp = performAllocation();
        }
 
        mHead = *reinterpret_cast_no_cast_align_warning<T**>(tmp);
        return tmp;
      }
 
      // does not actually deallocate but puts it in store.
      // make sure you have already called the destructor! e.g. with
      //  obj->~T();
      //  pool.deallocate(obj);
      void deallocate(T* obj) noexcept {
        *reinterpret_cast_no_cast_align_warning<T**>(obj) = mHead;
        mHead = obj;
      }
 
      // Adds an already allocated block of memory to the allocator. This allocator is from now on
      // responsible for freeing the data (with mem_free()). If the provided data is not large enough to
      // make use of, it is immediately freed. Otherwise it is reused and freed in the destructor.
      void addOrFree(void* ptr, const size_t numBytes) noexcept {
        // calculate number of available elements in ptr
        if (numBytes < ALIGNMENT + ALIGNED_SIZE) {
          // not enough data for at least one element. Free and return.
          ROBIN_HOOD_LOG("gaia::mem::mem_free");
          gaia::mem::mem_free(ptr);
        } else {
          ROBIN_HOOD_LOG("add to buffer");
          add(ptr, numBytes);
        }
      }
 
      void swap(BulkPoolAllocator<T, MinNumAllocs, MaxNumAllocs>& other) noexcept {
        using gaia::core::swap;
        swap(mHead, other.mHead);
        swap(mListForFree, other.mListForFree);
      }
 
    private:
      // iterates the list of allocated memory to calculate how many to alloc next.
      // Recalculating this each time saves us a size_t member.
      // This ignores the fact that memory blocks might have been added manually with addOrFree. In
      // practice, this should not matter much.
      GAIA_NODISCARD size_t calcNumElementsToAlloc() const noexcept {
        auto tmp = mListForFree;
        size_t numAllocs = MinNumAllocs;
 
        while (numAllocs * 2 <= MaxNumAllocs && tmp) {
          auto x = reinterpret_cast<T***>(tmp);
          tmp = *x;
          numAllocs *= 2;
        }
 
        return numAllocs;
      }
 
      // WARNING: Underflow if numBytes < ALIGNMENT! This is guarded in addOrFree().
      void add(void* ptr, const size_t numBytes) noexcept {
        const size_t numElements = (numBytes - ALIGNMENT) / ALIGNED_SIZE;
 
        auto data = reinterpret_cast<T**>(ptr);
 
        // link free list
        auto x = reinterpret_cast<T***>(data);
        *x = mListForFree;
        mListForFree = data;
 
        // create linked list for newly allocated data
        auto* const headT = reinterpret_cast_no_cast_align_warning<T*>(reinterpret_cast<char*>(ptr) + ALIGNMENT);
 
        auto* const head = reinterpret_cast<char*>(headT);
 
        // Visual Studio compiler automatically unrolls this loop, which is pretty cool
        for (size_t i = 0; i < numElements; ++i) {
          *reinterpret_cast_no_cast_align_warning<char**>(head + i * ALIGNED_SIZE) = head + (i + 1) * ALIGNED_SIZE;
        }
 
        // last one points to 0
        *reinterpret_cast_no_cast_align_warning<T**>(head + (numElements - 1) * ALIGNED_SIZE) = mHead;
        mHead = headT;
      }
 
      // Called when no memory is available (mHead == 0).
      // Don't inline this slow path.
      GAIA_NOINLINE T* performAllocation() {
        size_t const numElementsToAlloc = calcNumElementsToAlloc();
 
        // alloc new memory: [prev |T, T, ... T]
        size_t const bytes = ALIGNMENT + ALIGNED_SIZE * numElementsToAlloc;
        ROBIN_HOOD_LOG(
            "gaia::mem::mem_alloc %zu = %zu + %zu * %zu", bytes, ALIGNMENT, ALIGNED_SIZE, numElementsToAlloc);
        add(assertNotNull<std::bad_alloc>(gaia::mem::mem_alloc(bytes)), bytes);
        return mHead;
      }
 
      // enforce byte alignment of the T's
      static constexpr size_t ALIGNMENT =
          gaia::core::get_max(std::alignment_of<T>::value, std::alignment_of<T*>::value);
 
      static constexpr size_t ALIGNED_SIZE = ((sizeof(T) - 1) / ALIGNMENT + 1) * ALIGNMENT;
 
      static_assert(MinNumAllocs >= 1, "MinNumAllocs");
      static_assert(MaxNumAllocs >= MinNumAllocs, "MaxNumAllocs");
      static_assert(ALIGNED_SIZE >= sizeof(T*), "ALIGNED_SIZE");
      static_assert(0 == (ALIGNED_SIZE % sizeof(T*)), "ALIGNED_SIZE mod");
      static_assert(ALIGNMENT >= sizeof(T*), "ALIGNMENT");
 
      T* mHead{nullptr};
      T** mListForFree{nullptr};
    };
 
    template <typename T, size_t MinSize, size_t MaxSize, bool IsFlat>
    struct NodeAllocator;
 
    // dummy allocator that does nothing
    template <typename T, size_t MinSize, size_t MaxSize>
    struct NodeAllocator<T, MinSize, MaxSize, true> {
 
      // we are not using the data, so just free it.
      void addOrFree(void* ptr, size_t ROBIN_HOOD_UNUSED(numBytes) /*unused*/) noexcept {
        ROBIN_HOOD_LOG("gaia::mem::mem_free");
        gaia::mem::mem_free(ptr);
      }
    };
 
    template <typename T, size_t MinSize, size_t MaxSize>
    struct NodeAllocator<T, MinSize, MaxSize, false>: public BulkPoolAllocator<T, MinSize, MaxSize> {};
 
    namespace swappable {
      template <typename T>
      struct nothrow {
        static const bool value = std::is_nothrow_swappable<T>::value;
      };
    } // namespace swappable
 
  } // namespace detail
 
  struct is_transparent_tag {};
 
  // A custom pair implementation is used in the map because std::pair is not is_trivially_copyable,
  // which means it would not be allowed to be used in memcpy. This struct is copyable, which is
  // also tested.
  template <typename T1, typename T2>
  struct pair {
    using first_type = T1;
    using second_type = T2;
 
    template <
        typename U1 = T1, typename U2 = T2,
        typename = typename std::enable_if<
            std::is_default_constructible<U1>::value && std::is_default_constructible<U2>::value>::type>
    constexpr pair() noexcept(noexcept(U1()) && noexcept(U2())): first(), second() {}
 
    // pair constructors are explicit so we don't accidentally call this ctor when we don't have to.
    explicit constexpr pair(std::pair<T1, T2> const& o) noexcept(
        noexcept(T1(std::declval<T1 const&>())) && noexcept(T2(std::declval<T2 const&>()))):
        first(o.first), second(o.second) {}
 
    // pair constructors are explicit so we don't accidentally call this ctor when we don't have to.
    explicit constexpr pair(std::pair<T1, T2>&& o) noexcept(
        noexcept(T1(GAIA_MOV(std::declval<T1&&>()))) && noexcept(T2(GAIA_MOV(std::declval<T2&&>())))):
        first(GAIA_MOV(o.first)), second(GAIA_MOV(o.second)) {}
 
    constexpr pair(T1&& a, T2&& b) noexcept(
        noexcept(T1(GAIA_MOV(std::declval<T1&&>()))) && noexcept(T2(GAIA_MOV(std::declval<T2&&>())))):
        first(GAIA_MOV(a)), second(GAIA_MOV(b)) {}
 
    template <typename U1, typename U2>
    constexpr pair(U1&& a, U2&& b) noexcept(
        noexcept(T1(GAIA_FWD(std::declval<U1&&>()))) && noexcept(T2(GAIA_FWD(std::declval<U2&&>())))):
        first(GAIA_FWD(a)), second(GAIA_FWD(b)) {}
 
    template <typename... U1, typename... U2>
    // MSVC 2015 produces error "C2476: ‘constexpr’ constructor does not initialize all members"
    // if this constructor is constexpr
#if !ROBIN_HOOD(BROKEN_CONSTEXPR)
    constexpr
#endif
        pair(std::piecewise_construct_t /*unused*/, std::tuple<U1...> a, std::tuple<U2...> b) noexcept(noexcept(pair(
            std::declval<std::tuple<U1...>&>(), std::declval<std::tuple<U2...>&>(), std::index_sequence_for<U1...>(),
            std::index_sequence_for<U2...>()))):
        pair(a, b, std::index_sequence_for<U1...>(), std::index_sequence_for<U2...>()) {
    }
 
    // constructor called from the std::piecewise_construct_t ctor
    template <typename... U1, size_t... I1, typename... U2, size_t... I2>
    pair(std::tuple<U1...>& a, std::tuple<U2...>& b, std::index_sequence<I1...> /*unused*/, std::index_sequence<I2...> /*unused*/) noexcept(
        noexcept(T1(GAIA_FWD(std::get<I1>(std::declval<std::tuple<U1...>&>()))...)) &&
        noexcept(T2(GAIA_FWD(std::get<I2>(std::declval<std::tuple<U2...>&>()))...))):
        first(GAIA_FWD(std::get<I1>(a))...), second(GAIA_FWD(std::get<I2>(b))...) {
      // make visual studio compiler happy about warning about unused a & b.
      // Visual studio's pair implementation disables warning 4100.
      (void)a;
      (void)b;
    }
 
    void
    swap(pair<T1, T2>& o) noexcept((detail::swappable::nothrow<T1>::value) && (detail::swappable::nothrow<T2>::value)) {
      using gaia::core::swap;
      swap(first, o.first);
      swap(second, o.second);
    }
 
    T1 first; // NOLINT(misc-non-private-member-variables-in-classes)
    T2 second; // NOLINT(misc-non-private-member-variables-in-classes)
  };
 
  template <typename A, typename B>
  inline void
  swap(pair<A, B>& a, pair<A, B>& b) noexcept(noexcept(std::declval<pair<A, B>&>().swap(std::declval<pair<A, B>&>()))) {
    a.swap(b);
  }
 
  template <typename A, typename B>
  inline constexpr bool operator==(pair<A, B> const& x, pair<A, B> const& y) {
    return (x.first == y.first) && (x.second == y.second);
  }
  template <typename A, typename B>
  inline constexpr bool operator!=(pair<A, B> const& x, pair<A, B> const& y) {
    return !(x == y);
  }
  template <typename A, typename B>
  inline constexpr bool operator<(pair<A, B> const& x, pair<A, B> const& y) noexcept(
      noexcept(std::declval<A const&>() < std::declval<A const&>()) &&
      noexcept(std::declval<B const&>() < std::declval<B const&>())) {
    return x.first < y.first || (!(y.first < x.first) && x.second < y.second);
  }
  template <typename A, typename B>
  inline constexpr bool operator>(pair<A, B> const& x, pair<A, B> const& y) {
    return y < x;
  }
  template <typename A, typename B>
  inline constexpr bool operator<=(pair<A, B> const& x, pair<A, B> const& y) {
    return !(x > y);
  }
  template <typename A, typename B>
  inline constexpr bool operator>=(pair<A, B> const& x, pair<A, B> const& y) {
    return !(x < y);
  }
 
  inline size_t hash_bytes(void const* ptr, size_t len) noexcept {
    static constexpr uint64_t m = UINT64_C(0xc6a4a7935bd1e995);
    static constexpr uint64_t seed = UINT64_C(0xe17a1465);
    static constexpr unsigned int r = 47;
 
    auto const* const data64 = static_cast<uint64_t const*>(ptr);
    uint64_t h = seed ^ (len * m);
 
    size_t const n_blocks = len / 8;
    for (size_t i = 0; i < n_blocks; ++i) {
      auto k = detail::unaligned_load<uint64_t>(data64 + i);
 
      k *= m;
      k ^= k >> r;
      k *= m;
 
      h ^= k;
      h *= m;
    }
 
    auto const* const data8 = reinterpret_cast<uint8_t const*>(data64 + n_blocks);
    switch (len & 7U) {
      case 7:
        h ^= static_cast<uint64_t>(data8[6]) << 48U;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      case 6:
        h ^= static_cast<uint64_t>(data8[5]) << 40U;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      case 5:
        h ^= static_cast<uint64_t>(data8[4]) << 32U;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      case 4:
        h ^= static_cast<uint64_t>(data8[3]) << 24U;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      case 3:
        h ^= static_cast<uint64_t>(data8[2]) << 16U;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      case 2:
        h ^= static_cast<uint64_t>(data8[1]) << 8U;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      case 1:
        h ^= static_cast<uint64_t>(data8[0]);
        h *= m;
        ROBIN_HOOD(FALLTHROUGH); // FALLTHROUGH
      default:
        break;
    }
 
    h ^= h >> r;
 
    // not doing the final step here, because this will be done by keyToIdx anyways
    // h *= m;
    // h ^= h >> r;
    return static_cast<size_t>(h);
  }
 
  inline size_t hash_int(uint64_t x) noexcept {
    // tried lots of different hashes, let's stick with murmurhash3. It's simple, fast, well tested,
    // and doesn't need any special 128bit operations.
    x ^= x >> 33U;
    x *= UINT64_C(0xff51afd7ed558ccd);
    x ^= x >> 33U;
 
    // not doing the final step here, because this will be done by keyToIdx anyways
    // x *= UINT64_C(0xc4ceb9fe1a85ec53);
    // x ^= x >> 33U;
    return static_cast<size_t>(x);
  }
 
  template <typename T, typename Enable = void>
  struct hash {
    size_t operator()(T const& obj) const noexcept {
      return hash_bytes(&obj, sizeof(T));
    }
  };
 
  template <typename T>
  struct hash<T*> {
    size_t operator()(T* ptr) const noexcept {
      return hash_int(reinterpret_cast<detail::SizeT>(ptr));
    }
  };
 
#ifdef ROBIN_HOOD_STD_SMARTPOINTERS
  template <typename T>
  struct hash<std::unique_ptr<T>> {
    size_t operator()(std::unique_ptr<T> const& ptr) const noexcept {
      return hash_int(reinterpret_cast<detail::SizeT>(ptr.get()));
    }
  };
 
  template <typename T>
  struct hash<std::shared_ptr<T>> {
    size_t operator()(std::shared_ptr<T> const& ptr) const noexcept {
      return hash_int(reinterpret_cast<detail::SizeT>(ptr.get()));
    }
  };
#endif
 
  template <typename Enum>
  struct hash<Enum, typename std::enable_if<std::is_enum<Enum>::value>::type> {
    size_t operator()(Enum e) const noexcept {
      using Underlying = typename std::underlying_type<Enum>::type;
      return hash<Underlying>{}(static_cast<Underlying>(e));
    }
  };
 
  template <typename T>
  struct hash<T, typename std::enable_if<gaia::core::is_direct_hash_key_v<T>>::type> {
    size_t operator()(const T& obj) const noexcept {
      if constexpr (detail::has_hash<T>::value)
        return obj.hash;
      else
        return obj.hash();
    }
  };
 
#define ROBIN_HOOD_HASH_INT(T)                                                                                         \
  template <>                                                                                                          \
  struct hash<T> {                                                                                                     \
    size_t operator()(T const& obj) const noexcept {                                                                   \
      return hash_int(static_cast<uint64_t>(obj));                                                                     \
    }                                                                                                                  \
  }
 
#if defined(__GNUC__) && !defined(__clang__)
  #pragma GCC diagnostic push
  #pragma GCC diagnostic ignored "-Wuseless-cast"
#endif
  // see https://en.cppreference.com/w/cpp/utility/hash
  ROBIN_HOOD_HASH_INT(bool);
  ROBIN_HOOD_HASH_INT(char);
  ROBIN_HOOD_HASH_INT(signed char);
  ROBIN_HOOD_HASH_INT(unsigned char);
  ROBIN_HOOD_HASH_INT(char16_t);
  ROBIN_HOOD_HASH_INT(char32_t);
#if ROBIN_HOOD(HAS_NATIVE_WCHART)
  ROBIN_HOOD_HASH_INT(wchar_t);
#endif
  ROBIN_HOOD_HASH_INT(short);
  ROBIN_HOOD_HASH_INT(unsigned short);
  ROBIN_HOOD_HASH_INT(int);
  ROBIN_HOOD_HASH_INT(unsigned int);
  ROBIN_HOOD_HASH_INT(long);
  ROBIN_HOOD_HASH_INT(long long);
  ROBIN_HOOD_HASH_INT(unsigned long);
  ROBIN_HOOD_HASH_INT(unsigned long long);
#if defined(__GNUC__) && !defined(__clang__)
  #pragma GCC diagnostic pop
#endif
  namespace detail {
    template <typename, typename = void>
    struct has_is_transparent: public std::false_type {};
 
    template <typename T>
    struct has_is_transparent<T, std::void_t<decltype(T::is_transparent)>>: public std::true_type {};
 
    // using wrapper classes for hash and key_equal prevents the diamond problem when the same type
    // is used. see https://stackoverflow.com/a/28771920/48181
    template <typename T>
    struct WrapHash: public T {
      WrapHash() = default;
      explicit WrapHash(T const& o) noexcept(noexcept(T(std::declval<T const&>()))): T(o) {}
    };
 
    template <typename T>
    struct WrapKeyEqual: public T {
      WrapKeyEqual() = default;
      explicit WrapKeyEqual(T const& o) noexcept(noexcept(T(std::declval<T const&>()))): T(o) {}
    };
 
    // A highly optimized hashmap implementation, using the Robin Hood algorithm.
    //
    // In most cases, this map should be usable as a drop-in replacement for std::unordered_map, but
    // be about 2x faster in most cases and require much less allocations.
    //
    // This implementation uses the following memory layout:
    //
    // [Node, Node, ... Node | info, info, ... infoSentinel ]
    //
    // * Node: either a DataNode that directly has the std::pair<key, val> as member,
    //   or a DataNode with a pointer to std::pair<key,val>. Which DataNode representation to use
    //   depends on how fast the swap() operation is. Heuristically, this is automatically choosen
    //   based on sizeof(). there are always 2^n Nodes.
    //
    // * info: Each Node in the map has a corresponding info byte, so there are 2^n info bytes.
    //   Each byte is initialized to 0, meaning the corresponding Node is empty. Set to 1 means the
    //   corresponding node contains data. Set to 2 means the corresponding Node is filled, but it
    //   actually belongs to the previous position and was pushed out because that place is already
    //   taken.
    //
    // * infoSentinel: Sentinel byte set to 1, so that iterator's ++ can stop at end() without the
    //   need for a idx variable.
    //
    // According to STL, order of templates has effect on throughput. That's why I've moved the
    // boolean to the front.
    // https://www.reddit.com/r/cpp/comments/ahp6iu/compile_time_binary_size_reductions_and_cs_future/eeguck4/
    template <bool IsFlat, size_t MaxLoadFactor100, typename Key, typename T, typename Hash, typename KeyEqual>
    class Table:
        public WrapHash<Hash>,
        public WrapKeyEqual<KeyEqual>,
        detail::NodeAllocator<
            typename std::conditional<
                std::is_void<T>::value, Key,
                robin_hood::pair<typename std::conditional<IsFlat, Key, Key const>::type, T>>::type,
            4, 16384, IsFlat> {
    public:
      static constexpr bool is_flat = IsFlat;
      static constexpr bool is_map = !std::is_void<T>::value;
      static constexpr bool is_set = !is_map;
      static constexpr bool is_transparent = has_is_transparent<Hash>::value && has_is_transparent<KeyEqual>::value;
 
      using key_type = Key;
      using mapped_type = T;
      using value_type = typename std::conditional<
          is_set, Key, robin_hood::pair<typename std::conditional<is_flat, Key, Key const>::type, T>>::type;
      using size_type = size_t;
      using hasher = Hash;
      using key_equal = KeyEqual;
      using Self = Table<IsFlat, MaxLoadFactor100, key_type, mapped_type, hasher, key_equal>;
 
    private:
      static_assert(MaxLoadFactor100 > 10 && MaxLoadFactor100 < 100, "MaxLoadFactor100 needs to be >10 && < 100");
 
      using WHash = WrapHash<Hash>;
      using WKeyEqual = WrapKeyEqual<KeyEqual>;
 
      // configuration defaults
 
      // make sure we have 8 elements, needed to quickly rehash mInfo
      static constexpr size_t InitialNumElements = sizeof(uint64_t);
      static constexpr uint32_t InitialInfoNumBits = 5;
      static constexpr uint8_t InitialInfoInc = 1U << InitialInfoNumBits;
      static constexpr size_t InfoMask = InitialInfoInc - 1U;
      static constexpr uint8_t InitialInfoHashShift = 64U - InitialInfoNumBits;
      using DataPool = detail::NodeAllocator<value_type, 4, 16384, IsFlat>;
 
      // type needs to be wider than uint8_t.
      using InfoType = uint32_t;
 
      // DataNode ////////////////////////////////////////////////////////
 
      // Primary template for the data node. We have special implementations for small and big
      // objects. For large objects it is assumed that swap() is fairly slow, so we allocate these
      // on the heap so swap merely swaps a pointer.
      template <typename M, bool>
      class DataNode {};
 
      // Small: just allocate on the stack.
      template <typename M>
      class DataNode<M, true> final {
      public:
        template <typename... Args>
        explicit DataNode(M& ROBIN_HOOD_UNUSED(map) /*unused*/, Args&&... args) noexcept(
            noexcept(value_type(GAIA_FWD(args)...))): mData(GAIA_FWD(args)...) {}
 
        DataNode(M& ROBIN_HOOD_UNUSED(map) /*unused*/, DataNode<M, true>&& n) noexcept(
            std::is_nothrow_move_constructible<value_type>::value): mData(GAIA_MOV(n.mData)) {}
 
        // doesn't do anything
        void destroy(M& ROBIN_HOOD_UNUSED(map) /*unused*/) noexcept {}
        void destroyDoNotDeallocate() noexcept {}
 
        value_type const* operator->() const noexcept {
          return &mData;
        }
        value_type* operator->() noexcept {
          return &mData;
        }
 
        const value_type& operator*() const noexcept {
          return mData;
        }
 
        value_type& operator*() noexcept {
          return mData;
        }
 
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_map, typename VT::first_type&>::type getFirst() noexcept {
          return mData.first;
        }
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_set, VT&>::type getFirst() noexcept {
          return mData;
        }
 
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_map, typename VT::first_type const&>::type getFirst() const noexcept {
          return mData.first;
        }
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_set, VT const&>::type getFirst() const noexcept {
          return mData;
        }
 
        template <typename MT = mapped_type>
        GAIA_NODISCARD typename std::enable_if<is_map, MT&>::type getSecond() noexcept {
          return mData.second;
        }
 
        template <typename MT = mapped_type>
        GAIA_NODISCARD typename std::enable_if<is_set, MT const&>::type getSecond() const noexcept {
          return mData.second;
        }
 
        void
        swap(DataNode<M, true>& o) noexcept(noexcept(std::declval<value_type>().swap(std::declval<value_type>()))) {
          mData.swap(o.mData);
        }
 
      private:
        value_type mData;
      };
 
      // big object: allocate on heap.
      template <typename M>
      class DataNode<M, false> {
      public:
        template <typename... Args>
        explicit DataNode(M& map, Args&&... args): mData(map.allocate()) {
          ::new (static_cast<void*>(mData)) value_type(GAIA_FWD(args)...);
        }
 
        DataNode(M& ROBIN_HOOD_UNUSED(map) /*unused*/, DataNode<M, false>&& n) noexcept: mData(GAIA_MOV(n.mData)) {}
 
        void destroy(M& map) noexcept {
          // don't deallocate, just put it into list of datapool.
          mData->~value_type();
          map.deallocate(mData);
        }
 
        void destroyDoNotDeallocate() noexcept {
          mData->~value_type();
        }
 
        value_type const* operator->() const noexcept {
          return mData;
        }
 
        value_type* operator->() noexcept {
          return mData;
        }
 
        const value_type& operator*() const {
          return *mData;
        }
 
        value_type& operator*() {
          return *mData;
        }
 
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_map, typename VT::first_type&>::type getFirst() noexcept {
          return mData->first;
        }
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_set, VT&>::type getFirst() noexcept {
          return *mData;
        }
 
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_map, typename VT::first_type const&>::type getFirst() const noexcept {
          return mData->first;
        }
        template <typename VT = value_type>
        GAIA_NODISCARD typename std::enable_if<is_set, VT const&>::type getFirst() const noexcept {
          return *mData;
        }
 
        template <typename MT = mapped_type>
        GAIA_NODISCARD typename std::enable_if<is_map, MT&>::type getSecond() noexcept {
          return mData->second;
        }
 
        template <typename MT = mapped_type>
        GAIA_NODISCARD typename std::enable_if<is_map, MT const&>::type getSecond() const noexcept {
          return mData->second;
        }
 
        void swap(DataNode<M, false>& o) noexcept {
          using gaia::core::swap;
          swap(mData, o.mData);
        }
 
      private:
        value_type* mData;
      };
 
      using Node = DataNode<Self, IsFlat>;
 
      // helpers for insertKeyPrepareEmptySpot: extract first entry (only const required)
      GAIA_NODISCARD key_type const& getFirstConst(Node const& n) const noexcept {
        return n.getFirst();
      }
 
      // in case we have void mapped_type, we are not using a pair, thus we just route k through.
      // No need to disable this because it's just not used if not applicable.
      GAIA_NODISCARD key_type const& getFirstConst(key_type const& k) const noexcept {
        return k;
      }
 
      // in case we have non-void mapped_type, we have a standard robin_hood::pair
      template <typename Q = mapped_type>
      GAIA_NODISCARD typename std::enable_if<!std::is_void<Q>::value, key_type const&>::type
      getFirstConst(value_type const& vt) const noexcept {
        return vt.first;
      }
 
      // Cloner //////////////////////////////////////////////////////////
 
      template <typename M, bool UseMemcpy>
      struct Cloner;
 
      // fast path: Just copy data, without allocating anything.
      template <typename M>
      struct Cloner<M, true> {
        void operator()(M const& source, M& target) const {
          auto const* const src = reinterpret_cast<uint8_t const*>(source.mKeyVals);
          auto* tgt = reinterpret_cast<uint8_t*>(target.mKeyVals);
          auto const numElementsWithBuffer = target.calcNumElementsWithBuffer(target.mMask + 1);
          gaia::mem::copy_elements<uint8_t, false>(
              tgt, src, (uint32_t)target.calcNumBytesTotal(numElementsWithBuffer), 0, 0, 0);
        }
      };
 
      template <typename M>
      struct Cloner<M, false> {
        void operator()(M const& s, M& t) const {
          auto const numElementsWithBuffer = t.calcNumElementsWithBuffer(t.mMask + 1);
          gaia::mem::copy_elements<uint8_t, false>(
              t.mInfo, s.mInfo, (uint32_t)t.calcNumBytesInfo(numElementsWithBuffer), 0, 0, 0);
 
          for (size_t i = 0; i < numElementsWithBuffer; ++i) {
            if (t.mInfo[i]) {
              ::new (static_cast<void*>(t.mKeyVals + i)) Node(t, *s.mKeyVals[i]);
            }
          }
        }
      };
 
      // Destroyer ///////////////////////////////////////////////////////
 
      template <typename M, bool IsFlatAndTrivial>
      struct Destroyer {};
 
      template <typename M>
      struct Destroyer<M, true> {
        void nodes(M& m) const noexcept {
          m.mNumElements = 0;
        }
 
        void nodesDoNotDeallocate(M& m) const noexcept {
          m.mNumElements = 0;
        }
      };
 
      template <typename M>
      struct Destroyer<M, false> {
        void nodes(M& m) const noexcept {
          m.mNumElements = 0;
          // clear also resets mInfo to 0, that's sometimes not necessary.
          auto const numElementsWithBuffer = m.calcNumElementsWithBuffer(m.mMask + 1);
 
          for (size_t idx = 0; idx < numElementsWithBuffer; ++idx) {
            if (0 != m.mInfo[idx]) {
              Node& n = m.mKeyVals[idx];
              n.destroy(m);
              n.~Node();
            }
          }
        }
 
        void nodesDoNotDeallocate(M& m) const noexcept {
          m.mNumElements = 0;
          // clear also resets mInfo to 0, that's sometimes not necessary.
          auto const numElementsWithBuffer = m.calcNumElementsWithBuffer(m.mMask + 1);
          for (size_t idx = 0; idx < numElementsWithBuffer; ++idx) {
            if (0 != m.mInfo[idx]) {
              Node& n = m.mKeyVals[idx];
              n.destroyDoNotDeallocate();
              n.~Node();
            }
          }
        }
      };
 
      // Iter ////////////////////////////////////////////////////////////
 
      struct fast_forward_tag {};
 
      // generic iterator for both const_iterator and iterator.
      template <bool IsConst>
      // NOLINTNEXTLINE(hicpp-special-member-functions,cppcoreguidelines-special-member-functions)
      class Iter {
      private:
        using NodePtr = typename std::conditional<IsConst, Node const*, Node*>::type;
 
      public:
        using difference_type = std::ptrdiff_t;
        using value_type = typename Self::value_type;
        using reference = typename std::conditional<IsConst, value_type const&, value_type&>::type;
        using pointer = typename std::conditional<IsConst, value_type const*, value_type*>::type;
        using iterator_category = gaia::core::forward_iterator_tag;
 
        // default constructed iterator can be compared to itself, but WON'T return true when
        // compared to end().
        Iter() = default;
 
        // Rule of zero: nothing specified. The conversion constructor is only enabled for
        // iterator to const_iterator, so it doesn't accidentally work as a copy ctor.
 
        // Conversion constructor from iterator to const_iterator.
        template <bool OtherIsConst, typename = typename std::enable_if<IsConst && !OtherIsConst>::type>
        // NOLINTNEXTLINE(hicpp-explicit-conversions)
        Iter(Iter<OtherIsConst> const& other) noexcept: mKeyVals(other.mKeyVals), mInfo(other.mInfo) {}
 
        Iter(NodePtr valPtr, uint8_t const* infoPtr) noexcept: mKeyVals(valPtr), mInfo(infoPtr) {}
 
        Iter(NodePtr valPtr, uint8_t const* infoPtr, fast_forward_tag ROBIN_HOOD_UNUSED(tag) /*unused*/) noexcept:
            mKeyVals(valPtr), mInfo(infoPtr) {
          fastForward();
        }
 
        template <bool OtherIsConst, typename = typename std::enable_if<IsConst && !OtherIsConst>::type>
        Iter& operator=(Iter<OtherIsConst> const& other) noexcept {
          mKeyVals = other.mKeyVals;
          mInfo = other.mInfo;
          return *this;
        }
 
        // prefix increment. Undefined behavior if we are at end()!
        Iter& operator++() noexcept {
          ++mInfo;
          ++mKeyVals;
          fastForward();
          return *this;
        }
 
        Iter operator++(int) noexcept {
          Iter tmp = *this;
          ++(*this);
          return tmp;
        }
 
        reference operator*() const {
          return **mKeyVals;
        }
 
        pointer operator->() const {
          return &**mKeyVals;
        }
 
        template <bool O>
        bool operator==(Iter<O> const& o) const noexcept {
          return mKeyVals == o.mKeyVals;
        }
 
        template <bool O>
        bool operator!=(Iter<O> const& o) const noexcept {
          return mKeyVals != o.mKeyVals;
        }
 
      private:
        // fast forward to the next non-free info byte
        // I've tried a few variants that don't depend on intrinsics, but unfortunately they are
        // quite a bit slower than this one. So I've reverted that change again. See map_benchmark.
        void fastForward() noexcept {
          size_t n = 0;
          while (0U == (n = detail::unaligned_load<size_t>(mInfo))) {
            GAIA_MSVC_WARNING_PUSH()
            GAIA_MSVC_WARNING_DISABLE(6305)
            mInfo += sizeof(size_t);
            mKeyVals += sizeof(size_t);
            GAIA_MSVC_WARNING_POP()
          }
#if defined(ROBIN_HOOD_DISABLE_INTRINSICS)
          // we know for certain that within the next 8 bytes we'll find a non-zero one.
          if GAIA_UNLIKELY (0U == detail::unaligned_load<uint32_t>(mInfo)) {
            mInfo += 4;
            mKeyVals += 4;
          }
          if GAIA_UNLIKELY (0U == detail::unaligned_load<uint16_t>(mInfo)) {
            mInfo += 2;
            mKeyVals += 2;
          }
          if GAIA_UNLIKELY (0U == *mInfo) {
            mInfo += 1;
            mKeyVals += 1;
          }
#else
  #if GAIA_LITTLE_ENDIAN
          auto inc = ROBIN_HOOD_COUNT_TRAILING_ZEROES(n) / 8;
  #else
          auto inc = ROBIN_HOOD_COUNT_LEADING_ZEROES(n) / 8;
  #endif
          mInfo += inc;
          mKeyVals += inc;
#endif
        }
 
        friend class Table<IsFlat, MaxLoadFactor100, key_type, mapped_type, hasher, key_equal>;
        NodePtr mKeyVals{nullptr};
        uint8_t const* mInfo{nullptr};
      };
 
 
      // highly performance relevant code.
      // Lower bits are used for indexing into the array (2^n size)
      // The upper 1-5 bits need to be a reasonable good hash, to save comparisons.
      template <typename HashKey>
      void keyToIdx(HashKey&& key, size_t* idx, InfoType* info) const {
        auto h = static_cast<uint64_t>(WHash::operator()(key));
 
        // direct_hash_key is expected to be a proper hash. No additional hash tricks are required
        using HashKeyRaw = std::decay_t<HashKey>;
        if constexpr (!gaia::core::is_direct_hash_key_v<HashKeyRaw>) {
          // In addition to whatever hash is used, add another mul & shift so we get better hashing.
          // This serves as a bad hash prevention, if the given data is
          // badly mixed.
          h *= mHashMultiplier;
          h ^= h >> 33U;
        }
 
        // the lower InitialInfoNumBits are reserved for info.
        *info = mInfoInc + static_cast<InfoType>(h >> mInfoHashShift);
        *idx = (static_cast<size_t>(h)) & mMask;
      }
 
      // forwards the index by one, wrapping around at the end
      void next(InfoType* info, size_t* idx) const noexcept {
        *idx = *idx + 1;
        *info += mInfoInc;
      }
 
      void nextWhileLess(InfoType* info, size_t* idx) const noexcept {
        // unrolling this by hand did not bring any speedups.
        while (*info < mInfo[*idx]) {
          next(info, idx);
        }
      }
 
      // Shift everything up by one element. Tries to move stuff around.
      void shiftUp(size_t startIdx, size_t const insertion_idx) noexcept(std::is_nothrow_move_assignable<Node>::value) {
        auto idx = startIdx;
        ::new (static_cast<void*>(mKeyVals + idx)) Node(GAIA_MOV(mKeyVals[idx - 1]));
        while (--idx != insertion_idx) {
          mKeyVals[idx] = GAIA_MOV(mKeyVals[idx - 1]);
        }
 
        idx = startIdx;
        while (idx != insertion_idx) {
          mInfo[idx] = static_cast<uint8_t>(mInfo[idx - 1] + mInfoInc);
          if GAIA_UNLIKELY (mInfo[idx] + mInfoInc > 0xFF) {
            mMaxNumElementsAllowed = 0;
          }
          --idx;
        }
      }
 
      void shiftDown(size_t idx) noexcept(std::is_nothrow_move_assignable<Node>::value) {
        // until we find one that is either empty or has zero offset.
        // TODO(martinus) we don't need to move everything, just the last one for the same
        // bucket.
        mKeyVals[idx].destroy(*this);
 
        // until we find one that is either empty or has zero offset.
        while (mInfo[idx + 1] >= 2 * mInfoInc) {
          mInfo[idx] = static_cast<uint8_t>(mInfo[idx + 1] - mInfoInc);
          mKeyVals[idx] = GAIA_MOV(mKeyVals[idx + 1]);
          ++idx;
        }
 
        mInfo[idx] = 0;
        // don't destroy, we've moved it
        // mKeyVals[idx].destroy(*this);
        mKeyVals[idx].~Node();
      }
 
      // copy of find(), except that it returns iterator instead of const_iterator.
      template <typename Other>
      GAIA_NODISCARD size_t findIdx(Other const& key) const {
        size_t idx{};
        InfoType info{};
        keyToIdx(key, &idx, &info);
 
        do {
          // unrolling this twice gives a bit of a speedup. More unrolling did not help.
          if (info == mInfo[idx]) {
            if GAIA_LIKELY (WKeyEqual::operator()(key, mKeyVals[idx].getFirst())) {
              return idx;
            }
          }
          next(&info, &idx);
          if (info == mInfo[idx]) {
            if GAIA_LIKELY (WKeyEqual::operator()(key, mKeyVals[idx].getFirst())) {
              return idx;
            }
          }
          next(&info, &idx);
        } while (info <= mInfo[idx]);
 
        // nothing found!
        return mMask == 0 ? 0
                          : static_cast<size_t>(
                                gaia::core::distance(mKeyVals, reinterpret_cast_no_cast_align_warning<Node*>(mInfo)));
      }
 
      void cloneData(const Table& o) {
        Cloner<Table, IsFlat && ROBIN_HOOD_IS_TRIVIALLY_COPYABLE(Node)>()(o, *this);
      }
 
      // inserts a keyval that is guaranteed to be new, e.g. when the hashmap is resized.
      // @return True on success, false if something went wrong
      void insert_move(Node&& keyval) {
        // we don't retry, fail if overflowing
        // don't need to check max num elements
        if (0 == mMaxNumElementsAllowed && !try_increase_info()) {
          throwOverflowError();
        }
 
        size_t idx{};
        InfoType info{};
        keyToIdx(keyval.getFirst(), &idx, &info);
 
        // skip forward. Use <= because we are certain that the element is not there.
        while (info <= mInfo[idx]) {
          idx = idx + 1;
          info += mInfoInc;
        }
 
        // key not found, so we are now exactly where we want to insert it.
        auto const insertion_idx = idx;
        auto const insertion_info = static_cast<uint8_t>(info);
        if GAIA_UNLIKELY (insertion_info + mInfoInc > 0xFF) {
          mMaxNumElementsAllowed = 0;
        }
 
        // find an empty spot
        while (0 != mInfo[idx]) {
          next(&info, &idx);
        }
 
        auto& l = mKeyVals[insertion_idx];
        if (idx == insertion_idx) {
          ::new (static_cast<void*>(&l)) Node(GAIA_MOV(keyval));
        } else {
          shiftUp(idx, insertion_idx);
          l = GAIA_MOV(keyval);
        }
 
        // put at empty spot
        mInfo[insertion_idx] = insertion_info;
 
        ++mNumElements;
      }
 
    public:
      using iterator = Iter<false>;
      using const_iterator = Iter<true>;
 
      Table() noexcept(noexcept(Hash()) && noexcept(KeyEqual())): WHash(), WKeyEqual() {
        ROBIN_HOOD_TRACE("%p", this);
        GAIA_ASSERT(gaia::CheckEndianess());
      }
 
      // Creates an empty hash map. Nothing is allocated yet, this happens at the first insert.
      // This tremendously speeds up ctor & dtor of a map that never receives an element. The
      // penalty is payed at the first insert, and not before. Lookup of this empty map works
      // because everybody points to DummyInfoByte::b. parameter bucket_count is dictated by the
      // standard, but we can ignore it.
      explicit Table(
          size_t ROBIN_HOOD_UNUSED(bucket_count) /*unused*/, const Hash& h = Hash{},
          const KeyEqual& equal = KeyEqual{}) noexcept(noexcept(Hash(h)) && noexcept(KeyEqual(equal))):
          WHash(h), WKeyEqual(equal) {
        ROBIN_HOOD_TRACE("%p", this);
        GAIA_ASSERT(gaia::CheckEndianess());
      }
 
      template <typename Iter>
      Table(
          Iter first, Iter last, size_t ROBIN_HOOD_UNUSED(bucket_count) /*unused*/ = 0, const Hash& h = Hash{},
          const KeyEqual& equal = KeyEqual{}): WHash(h), WKeyEqual(equal) {
        ROBIN_HOOD_TRACE("%p", this);
        GAIA_ASSERT(gaia::CheckEndianess());
        insert(first, last);
      }
 
      Table(
          std::initializer_list<value_type> initlist, size_t ROBIN_HOOD_UNUSED(bucket_count) /*unused*/ = 0,
          const Hash& h = Hash{}, const KeyEqual& equal = KeyEqual{}): WHash(h), WKeyEqual(equal) {
        ROBIN_HOOD_TRACE("%p", this);
        GAIA_ASSERT(gaia::CheckEndianess());
        insert(initlist.begin(), initlist.end());
      }
 
      Table(Table&& o) noexcept:
          WHash(GAIA_MOV(static_cast<WHash&>(o))), WKeyEqual(GAIA_MOV(static_cast<WKeyEqual&>(o))),
          DataPool(GAIA_MOV(static_cast<DataPool&>(o))) {
        ROBIN_HOOD_TRACE("%p", this);
        if (o.mMask) {
          mHashMultiplier = GAIA_MOV(o.mHashMultiplier);
          mKeyVals = GAIA_MOV(o.mKeyVals);
          mInfo = GAIA_MOV(o.mInfo);
          mNumElements = GAIA_MOV(o.mNumElements);
          mMask = GAIA_MOV(o.mMask);
          mMaxNumElementsAllowed = GAIA_MOV(o.mMaxNumElementsAllowed);
          mInfoInc = GAIA_MOV(o.mInfoInc);
          mInfoHashShift = GAIA_MOV(o.mInfoHashShift);
          // set other's mask to 0 so its destructor won't do anything
          o.init();
        }
      }
 
      Table& operator=(Table&& o) noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        if (&o != this) {
          if (o.mMask) {
            // only move stuff if the other map actually has some data
            destroy();
            mHashMultiplier = GAIA_MOV(o.mHashMultiplier);
            mKeyVals = GAIA_MOV(o.mKeyVals);
            mInfo = GAIA_MOV(o.mInfo);
            mNumElements = GAIA_MOV(o.mNumElements);
            mMask = GAIA_MOV(o.mMask);
            mMaxNumElementsAllowed = GAIA_MOV(o.mMaxNumElementsAllowed);
            mInfoInc = GAIA_MOV(o.mInfoInc);
            mInfoHashShift = GAIA_MOV(o.mInfoHashShift);
            WHash::operator=(GAIA_MOV(static_cast<WHash&>(o)));
            WKeyEqual::operator=(GAIA_MOV(static_cast<WKeyEqual&>(o)));
            DataPool::operator=(GAIA_MOV(static_cast<DataPool&>(o)));
 
            o.init();
 
          } else {
            // nothing in the other map => just clear us.
            clear();
          }
        }
        return *this;
      }
 
      Table(const Table& o):
          WHash(static_cast<const WHash&>(o)), WKeyEqual(static_cast<const WKeyEqual&>(o)),
          DataPool(static_cast<const DataPool&>(o)) {
        ROBIN_HOOD_TRACE("%p", this);
        if (!o.empty()) {
          // not empty: create an exact copy. it is also possible to just iterate through all
          // elements and insert them, but copying is probably faster.
 
          auto const numElementsWithBuffer = calcNumElementsWithBuffer(o.mMask + 1);
          auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);
 
          ROBIN_HOOD_LOG("gaia::mem::mem_alloc %zu = calcNumBytesTotal(%zu)", numBytesTotal, numElementsWithBuffer);
          mHashMultiplier = o.mHashMultiplier;
          mKeyVals = static_cast<Node*>(detail::assertNotNull<std::bad_alloc>(gaia::mem::mem_alloc(numBytesTotal)));
          // no need for calloc because clonData does memcpy
          mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);
          mNumElements = o.mNumElements;
          mMask = o.mMask;
          mMaxNumElementsAllowed = o.mMaxNumElementsAllowed;
          mInfoInc = o.mInfoInc;
          mInfoHashShift = o.mInfoHashShift;
          cloneData(o);
        }
      }
 
      // Creates a copy of the given map. Copy constructor of each entry is used.
      // Not sure why clang-tidy thinks this doesn't handle self assignment, it does
      // NOLINTNEXTLINE(bugprone-unhandled-self-assignment,cert-oop54-cpp)
      Table& operator=(Table const& o) {
        ROBIN_HOOD_TRACE("%p", this);
        if (&o == this) {
          // prevent assigning of itself
          return *this;
        }
 
        // we keep using the old allocator and not assign the new one, because we want to keep
        // the memory available. when it is the same size.
        if (o.empty()) {
          if (0 == mMask) {
            // nothing to do, we are empty too
            return *this;
          }
 
          // not empty: destroy what we have there
          // clear also resets mInfo to 0, that's sometimes not necessary.
          destroy();
          init();
          WHash::operator=(static_cast<const WHash&>(o));
          WKeyEqual::operator=(static_cast<const WKeyEqual&>(o));
          DataPool::operator=(static_cast<DataPool const&>(o));
 
          return *this;
        }
 
        // clean up old stuff
        Destroyer<Self, IsFlat && std::is_trivially_destructible<Node>::value>{}.nodes(*this);
 
        if (mMask != o.mMask) {
          // no luck: we don't have the same array size allocated, so we need to realloc.
          if (0 != mMask) {
            // only deallocate if we actually have data!
            ROBIN_HOOD_LOG("gaia::mem::mem_free");
            gaia::mem::mem_free(mKeyVals);
          }
 
          auto const numElementsWithBuffer = calcNumElementsWithBuffer(o.mMask + 1);
          auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);
          ROBIN_HOOD_LOG("gaia::mem::mem_alloc %zu = calcNumBytesTotal(%zu)", numBytesTotal, numElementsWithBuffer);
          mKeyVals = static_cast<Node*>(detail::assertNotNull<std::bad_alloc>(gaia::mem::mem_alloc(numBytesTotal)));
 
          // no need for calloc here because cloneData performs a memcpy.
          mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);
          // sentinel is set in cloneData
        }
        WHash::operator=(static_cast<const WHash&>(o));
        WKeyEqual::operator=(static_cast<const WKeyEqual&>(o));
        DataPool::operator=(static_cast<DataPool const&>(o));
        mHashMultiplier = o.mHashMultiplier;
        mNumElements = o.mNumElements;
        mMask = o.mMask;
        mMaxNumElementsAllowed = o.mMaxNumElementsAllowed;
        mInfoInc = o.mInfoInc;
        mInfoHashShift = o.mInfoHashShift;
        cloneData(o);
 
        return *this;
      }
 
      // Swaps everything between the two maps.
      void swap(Table& o) {
        ROBIN_HOOD_TRACE("%p", this);
        using gaia::core::swap;
        swap(o, *this);
      }
 
      // Clears all data, without resizing.
      void clear() {
        ROBIN_HOOD_TRACE("%p", this);
        if (empty()) {
          // don't do anything! also important because we don't want to write to
          // DummyInfoByte::b, even though we would just write 0 to it.
          return;
        }
 
        Destroyer<Self, IsFlat && std::is_trivially_destructible<Node>::value>{}.nodes(*this);
 
        auto const numElementsWithBuffer = calcNumElementsWithBuffer(mMask + 1);
        // clear everything, then set the sentinel again
        uint8_t const z = 0;
        gaia::core::fill(mInfo, mInfo + calcNumBytesInfo(numElementsWithBuffer), z);
        mInfo[numElementsWithBuffer] = 1;
 
        mInfoInc = InitialInfoInc;
        mInfoHashShift = InitialInfoHashShift;
      }
 
      // Destroys the map and all it's contents.
      ~Table() {
        ROBIN_HOOD_TRACE("%p", this);
        destroy();
      }
 
      // Checks if both tables contain the same entries. Order is irrelevant.
      bool operator==(const Table& other) const {
        ROBIN_HOOD_TRACE("%p", this);
        if (other.size() != size()) {
          return false;
        }
        for (auto const& otherEntry: other) {
          if (!has(otherEntry)) {
            return false;
          }
        }
 
        return true;
      }
 
      bool operator!=(const Table& other) const {
        ROBIN_HOOD_TRACE("%p", this);
        return !operator==(other);
      }
 
      template <typename Q = mapped_type>
      typename std::enable_if<!std::is_void<Q>::value, Q&>::type operator[](const key_type& key) {
        ROBIN_HOOD_TRACE("%p", this);
        auto idxAndState = insertKeyPrepareEmptySpot(key);
        switch (idxAndState.second) {
          case InsertionState::key_found:
            break;
 
          case InsertionState::new_node:
            ::new (static_cast<void*>(&mKeyVals[idxAndState.first]))
                Node(*this, std::piecewise_construct, std::forward_as_tuple(key), std::forward_as_tuple());
            break;
 
          case InsertionState::overwrite_node:
            mKeyVals[idxAndState.first] =
                Node(*this, std::piecewise_construct, std::forward_as_tuple(key), std::forward_as_tuple());
            break;
 
          case InsertionState::overflow_error:
            throwOverflowError();
        }
 
        return mKeyVals[idxAndState.first].getSecond();
      }
 
      template <typename Q = mapped_type>
      typename std::enable_if<!std::is_void<Q>::value, Q&>::type operator[](key_type&& key) {
        ROBIN_HOOD_TRACE("%p", this);
        auto idxAndState = insertKeyPrepareEmptySpot(key);
        switch (idxAndState.second) {
          case InsertionState::key_found:
            break;
 
          case InsertionState::new_node:
            ::new (static_cast<void*>(&mKeyVals[idxAndState.first]))
                Node(*this, std::piecewise_construct, std::forward_as_tuple(GAIA_MOV(key)), std::forward_as_tuple());
            break;
 
          case InsertionState::overwrite_node:
            mKeyVals[idxAndState.first] =
                Node(*this, std::piecewise_construct, std::forward_as_tuple(GAIA_MOV(key)), std::forward_as_tuple());
            break;
 
          case InsertionState::overflow_error:
            throwOverflowError();
        }
 
        return mKeyVals[idxAndState.first].getSecond();
      }
 
      template <typename Iter>
      void insert(Iter first, Iter last) {
        for (; first != last; ++first) {
          // value_type ctor needed because this might be called with std::pair's
          insert(value_type(*first));
        }
      }
 
      void insert(std::initializer_list<value_type> ilist) {
        for (auto&& vt: ilist) {
          insert(GAIA_MOV(vt));
        }
      }
 
      template <typename... Args>
      std::pair<iterator, bool> emplace(Args&&... args) {
        ROBIN_HOOD_TRACE("%p", this);
        Node n{*this, GAIA_FWD(args)...};
        auto idxAndState = insertKeyPrepareEmptySpot(getFirstConst(n));
        switch (idxAndState.second) {
          case InsertionState::key_found:
            n.destroy(*this);
            break;
 
          case InsertionState::new_node:
            ::new (static_cast<void*>(&mKeyVals[idxAndState.first])) Node(*this, GAIA_MOV(n));
            break;
 
          case InsertionState::overwrite_node:
            mKeyVals[idxAndState.first] = GAIA_MOV(n);
            break;
 
          case InsertionState::overflow_error:
            n.destroy(*this);
            throwOverflowError();
            break;
        }
 
        return std::make_pair(
            iterator(mKeyVals + idxAndState.first, mInfo + idxAndState.first),
            InsertionState::key_found != idxAndState.second);
      }
 
      template <typename... Args>
      iterator emplace_hint(const_iterator position, Args&&... args) {
        (void)position;
        return emplace(GAIA_FWD(args)...).first;
      }
 
      template <typename... Args>
      std::pair<iterator, bool> try_emplace(const key_type& key, Args&&... args) {
        return try_emplace_impl(key, GAIA_FWD(args)...);
      }
 
      template <typename... Args>
      std::pair<iterator, bool> try_emplace(key_type&& key, Args&&... args) {
        return try_emplace_impl(GAIA_MOV(key), GAIA_FWD(args)...);
      }
 
      template <typename... Args>
      iterator try_emplace(const_iterator hint, const key_type& key, Args&&... args) {
        (void)hint;
        return try_emplace_impl(key, GAIA_FWD(args)...).first;
      }
 
      template <typename... Args>
      iterator try_emplace(const_iterator hint, key_type&& key, Args&&... args) {
        (void)hint;
        return try_emplace_impl(GAIA_MOV(key), GAIA_FWD(args)...).first;
      }
 
      template <typename Mapped>
      std::pair<iterator, bool> insert_or_assign(const key_type& key, Mapped&& obj) {
        return insertOrAssignImpl(key, GAIA_FWD(obj));
      }
 
      template <typename Mapped>
      std::pair<iterator, bool> insert_or_assign(key_type&& key, Mapped&& obj) {
        return insertOrAssignImpl(GAIA_MOV(key), GAIA_FWD(obj));
      }
 
      template <typename Mapped>
      iterator insert_or_assign(const_iterator hint, const key_type& key, Mapped&& obj) {
        (void)hint;
        return insertOrAssignImpl(key, GAIA_FWD(obj)).first;
      }
 
      template <typename Mapped>
      iterator insert_or_assign(const_iterator hint, key_type&& key, Mapped&& obj) {
        (void)hint;
        return insertOrAssignImpl(GAIA_MOV(key), GAIA_FWD(obj)).first;
      }
 
      std::pair<iterator, bool> insert(const value_type& keyval) {
        ROBIN_HOOD_TRACE("%p", this);
        return emplace(keyval);
      }
 
      iterator insert(const_iterator hint, const value_type& keyval) {
        (void)hint;
        return emplace(keyval).first;
      }
 
      std::pair<iterator, bool> insert(value_type&& keyval) {
        return emplace(GAIA_MOV(keyval));
      }
 
      iterator insert(const_iterator hint, value_type&& keyval) {
        (void)hint;
        return emplace(GAIA_MOV(keyval)).first;
      }
 
      // Returns 1 if key is found, 0 otherwise.
      GAIA_NODISCARD size_t count(const key_type& key) const {
        ROBIN_HOOD_TRACE("%p", this);
        auto kv = mKeyVals + findIdx(key);
        if (kv != reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
          return 1;
        }
        return 0;
      }
 
      template <typename OtherKey, typename Self_ = Self>
      GAIA_NODISCARD typename std::enable_if<Self_::is_transparent, size_t>::type count(const OtherKey& key) const {
        ROBIN_HOOD_TRACE("%p", this);
        auto kv = mKeyVals + findIdx(key);
        if (kv != reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
          return 1;
        }
        return 0;
      }
 
      GAIA_NODISCARD bool contains(const key_type& key) const {
        return 1U == count(key);
      }
 
      template <typename OtherKey, typename Self_ = Self>
      GAIA_NODISCARD typename std::enable_if<Self_::is_transparent, bool>::type contains(const OtherKey& key) const {
        return 1U == count(key);
      }
 
      // Returns a reference to the value found for key.
      // Throws std::out_of_range if element cannot be found
      template <typename Q = mapped_type>
      GAIA_NODISCARD typename std::enable_if<!std::is_void<Q>::value, Q&>::type at(key_type const& key) {
        ROBIN_HOOD_TRACE("%p", this);
        auto kv = mKeyVals + findIdx(key);
        if (kv == reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
          doThrow<ROBIN_HOOD_STD_OUT_OF_RANGE>("key not found");
        }
        return kv->getSecond();
      }
 
      // Returns a reference to the value found for key.
      // Throws std::out_of_range if element cannot be found
      template <typename Q = mapped_type>
      GAIA_NODISCARD typename std::enable_if<!std::is_void<Q>::value, Q const&>::type at(key_type const& key) const {
        ROBIN_HOOD_TRACE("%p", this);
        auto kv = mKeyVals + findIdx(key);
        if (kv == reinterpret_cast_no_cast_align_warning<Node*>(mInfo)) {
          doThrow<ROBIN_HOOD_STD_OUT_OF_RANGE>("key not found");
        }
        return kv->getSecond();
      }
 
      GAIA_NODISCARD const_iterator find(const key_type& key) const {
        ROBIN_HOOD_TRACE("%p", this);
        const size_t idx = findIdx(key);
        return const_iterator{mKeyVals + idx, mInfo + idx};
      }
 
      template <typename OtherKey>
      GAIA_NODISCARD const_iterator find(const OtherKey& key, is_transparent_tag /*unused*/) const {
        ROBIN_HOOD_TRACE("%p", this);
        const size_t idx = findIdx(key);
        return const_iterator{mKeyVals + idx, mInfo + idx};
      }
 
      template <typename OtherKey, typename Self_ = Self>
      GAIA_NODISCARD typename std::enable_if<Self_::is_transparent, const_iterator>::type
      find(const OtherKey& key) const {
        ROBIN_HOOD_TRACE("%p", this);
        const size_t idx = findIdx(key);
        return const_iterator{mKeyVals + idx, mInfo + idx};
      }
 
      GAIA_NODISCARD iterator find(const key_type& key) {
        ROBIN_HOOD_TRACE("%p", this);
        const size_t idx = findIdx(key);
        return iterator{mKeyVals + idx, mInfo + idx};
      }
 
      template <typename OtherKey>
      GAIA_NODISCARD iterator find(const OtherKey& key, is_transparent_tag /*unused*/) {
        ROBIN_HOOD_TRACE("%p", this);
        const size_t idx = findIdx(key);
        return iterator{mKeyVals + idx, mInfo + idx};
      }
 
      template <typename OtherKey, typename Self_ = Self>
      GAIA_NODISCARD typename std::enable_if<Self_::is_transparent, iterator>::type find(const OtherKey& key) {
        ROBIN_HOOD_TRACE("%p", this);
        const size_t idx = findIdx(key);
        return iterator{mKeyVals + idx, mInfo + idx};
      }
 
      GAIA_NODISCARD iterator begin() {
        ROBIN_HOOD_TRACE("%p", this);
        if (empty()) {
          return end();
        }
        return iterator(mKeyVals, mInfo, fast_forward_tag{});
      }
      GAIA_NODISCARD const_iterator begin() const {
        ROBIN_HOOD_TRACE("%p", this);
        return cbegin();
      }
      GAIA_NODISCARD const_iterator cbegin() const {
        ROBIN_HOOD_TRACE("%p", this);
        if (empty()) {
          return cend();
        }
        return const_iterator(mKeyVals, mInfo, fast_forward_tag{});
      }
 
      GAIA_NODISCARD iterator end() {
        ROBIN_HOOD_TRACE("%p", this);
        // no need to supply valid info pointer: end() must not be dereferenced, and only node
        // pointer is compared.
        return iterator{reinterpret_cast_no_cast_align_warning<Node*>(mInfo), nullptr};
      }
      GAIA_NODISCARD const_iterator end() const {
        ROBIN_HOOD_TRACE("%p", this);
        return cend();
      }
      GAIA_NODISCARD const_iterator cend() const {
        ROBIN_HOOD_TRACE("%p", this);
        return const_iterator{reinterpret_cast_no_cast_align_warning<Node*>(mInfo), nullptr};
      }
 
      iterator erase(const_iterator pos) {
        ROBIN_HOOD_TRACE("%p", this);
        // its safe to perform const cast here
        // NOLINTNEXTLINE(cppcoreguidelines-pro-type-const-cast)
        return erase(iterator{const_cast<Node*>(pos.mKeyVals), const_cast<uint8_t*>(pos.mInfo)});
      }
 
      // Erases element at pos, returns iterator to the next element.
      iterator erase(iterator pos) {
        ROBIN_HOOD_TRACE("%p", this);
        // we assume that pos always points to a valid entry, and not end().
        auto const idx = static_cast<size_t>(pos.mKeyVals - mKeyVals);
 
        shiftDown(idx);
        --mNumElements;
 
        if (*pos.mInfo) {
          // we've backward shifted, return this again
          return pos;
        }
 
        // no backward shift, return next element
        return ++pos;
      }
 
      size_t erase(const key_type& key) {
        ROBIN_HOOD_TRACE("%p", this);
        size_t idx{};
        InfoType info{};
        keyToIdx(key, &idx, &info);
 
        // check while info matches with the source idx
        do {
          if (info == mInfo[idx] && WKeyEqual::operator()(key, mKeyVals[idx].getFirst())) {
            shiftDown(idx);
            --mNumElements;
            return 1;
          }
          next(&info, &idx);
        } while (info <= mInfo[idx]);
 
        // nothing found to delete
        return 0;
      }
 
      // reserves space for the specified number of elements. Makes sure the old data fits.
      // exactly the same as reserve(c).
      void rehash(size_t c) {
        // forces a reserve
        reserve(c, true);
      }
 
      // reserves space for the specified number of elements. Makes sure the old data fits.
      // Exactly the same as rehash(c). Use rehash(0) to shrink to fit.
      void reserve(size_t c) {
        // reserve, but don't force rehash
        reserve(c, false);
      }
 
      // If possible reallocates the map to a smaller one. This frees the underlying table.
      // Does not do anything if load_factor is too large for decreasing the table's size.
      void compact() {
        ROBIN_HOOD_TRACE("%p", this);
        auto newSize = InitialNumElements;
        while (calcMaxNumElementsAllowed(newSize) < mNumElements && newSize != 0) {
          newSize *= 2;
        }
        if GAIA_UNLIKELY (newSize == 0) {
          throwOverflowError();
        }
 
        ROBIN_HOOD_LOG("newSize > mMask + 1: %zu > %zu + 1", newSize, mMask);
 
        // only actually do anything when the new size is bigger than the old one. This prevents to
        // continuously allocate for each reserve() call.
        if (newSize < mMask + 1) {
          rehashPowerOfTwo(newSize, true);
        }
      }
 
      constexpr uint32_t bytes() const noexcept {
        if constexpr (is_map) {
          return sizeof(key_type) + sizeof(value_type);
        } else {
          return sizeof(key_type);
        }
      }
 
      template <typename Serializer>
      void save(Serializer& s) const {
        const auto cnt = (uint32_t)size();
        s.save(cnt);
 
        if constexpr (is_map) {
          for (auto& p: *this) {
            ::gaia::ser::save(s, p.first);
            ::gaia::ser::save(s, p.second);
          }
        } else {
          for (auto& p: *this) {
            ::gaia::ser::save(s, p);
          }
        }
      }
 
      template <typename Serializer>
      void load(Serializer& s) {
        // Clear old data
        clear();
 
        uint32_t cnt = 0;
        s.load(cnt);
 
        if constexpr (is_map) {
          for (uint32_t i = 0; i < cnt; ++i) {
            typename Table::key_type key;
            typename Table::mapped_type value;
            ::gaia::ser::load(s, key);
            ::gaia::ser::load(s, value);
            insert({key, value});
          }
        } else {
          for (uint32_t i = 0; i < cnt; ++i) {
            typename Table::key_type key;
            ::gaia::ser::load(s, key);
            insert(key);
          }
        }
      }
 
      GAIA_NODISCARD size_type capacity() const noexcept {
        return calcMaxNumElementsAllowed(mMask);
      }
 
      GAIA_NODISCARD size_type size() const noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        return mNumElements;
      }
 
      GAIA_NODISCARD size_type max_size() const noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        return static_cast<size_type>(-1);
      }
 
      GAIA_NODISCARD bool empty() const noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        return 0 == mNumElements;
      }
 
      GAIA_NODISCARD float max_load_factor() const noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        return MaxLoadFactor100 / 100.0f;
      }
 
      // Average number of elements per bucket. Since we allow only 1 per bucket
      GAIA_NODISCARD float load_factor() const noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        return static_cast<float>(size()) / static_cast<float>(mMask + 1);
      }
 
      GAIA_NODISCARD size_t mask() const noexcept {
        ROBIN_HOOD_TRACE("%p", this);
        return mMask;
      }
 
      GAIA_NODISCARD size_t calcMaxNumElementsAllowed(size_t maxElements) const noexcept {
        if GAIA_LIKELY (maxElements <= (size_t(-1) / 100)) {
          return maxElements * MaxLoadFactor100 / 100;
        }
 
        // we might be a bit inprecise, but since maxElements is quite large that doesn't matter
        return (maxElements / 100) * MaxLoadFactor100;
      }
 
      GAIA_NODISCARD size_t calcNumBytesInfo(size_t numElements) const noexcept {
        // we add a uint64_t, which houses the sentinel (first byte) and padding so we can load
        // 64bit types.
        return numElements + sizeof(uint64_t);
      }
 
      GAIA_NODISCARD size_t calcNumElementsWithBuffer(size_t numElements) const noexcept {
        auto maxNumElementsAllowed = calcMaxNumElementsAllowed(numElements);
        return numElements + gaia::core::get_min(maxNumElementsAllowed, (static_cast<size_t>(0xFF)));
      }
 
      // calculation only allowed for 2^n values
      GAIA_NODISCARD size_t calcNumBytesTotal(size_t numElements) const {
#if ROBIN_HOOD(BITNESS) == 64
        return (numElements * sizeof(Node)) + calcNumBytesInfo(numElements);
#else
        // make sure we're doing 64bit operations, so we are at least safe against 32bit overflows.
        auto const ne = static_cast<uint64_t>(numElements);
        auto const s = static_cast<uint64_t>(sizeof(Node));
        auto const infos = static_cast<uint64_t>(calcNumBytesInfo(numElements));
 
        auto const total64 = ne * s + infos;
        auto const total = static_cast<size_t>(total64);
 
        if GAIA_UNLIKELY (static_cast<uint64_t>(total) != total64) {
          throwOverflowError();
        }
        return total;
#endif
      }
 
    private:
      template <typename Q = mapped_type>
      GAIA_NODISCARD typename std::enable_if<!std::is_void<Q>::value, bool>::type has(const value_type& e) const {
        ROBIN_HOOD_TRACE("%p", this);
        auto it = find(e.first);
        return it != end() && it->second == e.second;
      }
 
      template <typename Q = mapped_type>
      GAIA_NODISCARD typename std::enable_if<std::is_void<Q>::value, bool>::type has(const value_type& e) const {
        ROBIN_HOOD_TRACE("%p", this);
        return find(e) != end();
      }
 
      void reserve(size_t c, bool forceRehash) {
        ROBIN_HOOD_TRACE("%p", this);
        auto const minElementsAllowed = gaia::core::get_max(c, mNumElements);
        auto newSize = InitialNumElements;
        while (calcMaxNumElementsAllowed(newSize) < minElementsAllowed && newSize != 0) {
          newSize *= 2;
        }
        if GAIA_UNLIKELY (newSize == 0) {
          throwOverflowError();
        }
 
        ROBIN_HOOD_LOG("newSize > mMask + 1: %zu > %zu + 1", newSize, mMask);
 
        // only actually do anything when the new size is bigger than the old one. This prevents to
        // continuously allocate for each reserve() call.
        if (forceRehash || newSize > mMask + 1) {
          rehashPowerOfTwo(newSize, false);
        }
      }
 
      // reserves space for at least the specified number of elements.
      // only works if numBuckets if power of two
      // True on success, false otherwise
      void rehashPowerOfTwo(size_t numBuckets, bool forceFree) {
        ROBIN_HOOD_TRACE("%p", this);
 
        Node* const oldKeyVals = mKeyVals;
        uint8_t const* const oldInfo = mInfo;
 
        const size_t oldMaxElementsWithBuffer = calcNumElementsWithBuffer(mMask + 1);
 
        // resize operation: move stuff
        initData(numBuckets);
        if (oldMaxElementsWithBuffer > 1) {
          for (size_t i = 0; i < oldMaxElementsWithBuffer; ++i) {
            if (oldInfo[i] != 0) {
              // might throw an exception, which is really bad since we are in the middle of
              // moving stuff.
              insert_move(GAIA_MOV(oldKeyVals[i]));
              // destroy the node but DON'T destroy the data.
              oldKeyVals[i].~Node();
            }
          }
 
          // this check is not necessary as it's guarded by the previous if, but it helps
          // silence g++'s overeager "attempt to free a non-heap object 'map'
          // [-Werror=free-nonheap-object]" warning.
          if (oldKeyVals != reinterpret_cast_no_cast_align_warning<Node*>(&mMask)) {
            // don't destroy old data: put it into the pool instead
            if (forceFree) {
              gaia::mem::mem_free(oldKeyVals);
            } else {
              DataPool::addOrFree(oldKeyVals, calcNumBytesTotal(oldMaxElementsWithBuffer));
            }
          }
        }
      }
 
      GAIA_NOINLINE void throwOverflowError() const {
#if ROBIN_HOOD(HAS_EXCEPTIONS)
        throw std::overflow_error("robin_hood::map overflow");
#else
        abort();
#endif
      }
 
      template <typename OtherKey, typename... Args>
      std::pair<iterator, bool> try_emplace_impl(OtherKey&& key, Args&&... args) {
        ROBIN_HOOD_TRACE("%p", this);
        auto idxAndState = insertKeyPrepareEmptySpot(key);
        switch (idxAndState.second) {
          case InsertionState::key_found:
            break;
 
          case InsertionState::new_node:
            ::new (static_cast<void*>(&mKeyVals[idxAndState.first])) Node(
                *this, std::piecewise_construct, std::forward_as_tuple(GAIA_FWD(key)),
                std::forward_as_tuple(GAIA_FWD(args)...));
            break;
 
          case InsertionState::overwrite_node:
            mKeyVals[idxAndState.first] = Node(
                *this, std::piecewise_construct, std::forward_as_tuple(GAIA_FWD(key)),
                std::forward_as_tuple(GAIA_FWD(args)...));
            break;
 
          case InsertionState::overflow_error:
            throwOverflowError();
            break;
        }
 
        return std::make_pair(
            iterator(mKeyVals + idxAndState.first, mInfo + idxAndState.first),
            InsertionState::key_found != idxAndState.second);
      }
 
      template <typename OtherKey, typename Mapped>
      std::pair<iterator, bool> insertOrAssignImpl(OtherKey&& key, Mapped&& obj) {
        ROBIN_HOOD_TRACE("%p", this);
        auto idxAndState = insertKeyPrepareEmptySpot(key);
        switch (idxAndState.second) {
          case InsertionState::key_found:
            mKeyVals[idxAndState.first].getSecond() = GAIA_FWD(obj);
            break;
 
          case InsertionState::new_node:
            ::new (static_cast<void*>(&mKeyVals[idxAndState.first])) Node(
                *this, std::piecewise_construct, std::forward_as_tuple(GAIA_FWD(key)),
                std::forward_as_tuple(GAIA_FWD(obj)));
            break;
 
          case InsertionState::overwrite_node:
            mKeyVals[idxAndState.first] = Node(
                *this, std::piecewise_construct, std::forward_as_tuple(GAIA_FWD(key)),
                std::forward_as_tuple(GAIA_FWD(obj)));
            break;
 
          case InsertionState::overflow_error:
            throwOverflowError();
            break;
        }
 
        return std::make_pair(
            iterator(mKeyVals + idxAndState.first, mInfo + idxAndState.first),
            InsertionState::key_found != idxAndState.second);
      }
 
      void initData(size_t max_elements) {
        mNumElements = 0;
        mMask = max_elements - 1;
        mMaxNumElementsAllowed = calcMaxNumElementsAllowed(max_elements);
 
        auto const numElementsWithBuffer = calcNumElementsWithBuffer(max_elements);
 
        // malloc & zero mInfo. Faster than calloc everything.
        auto const numBytesTotal = calcNumBytesTotal(numElementsWithBuffer);
 
        ROBIN_HOOD_LOG("gaia::mem::mem_alloc %zu = calcNumBytesTotal(%zu)", numBytesTotal, numElementsWithBuffer);
        mKeyVals = reinterpret_cast<Node*>(detail::assertNotNull<std::bad_alloc>(gaia::mem::mem_alloc(numBytesTotal)));
        mInfo = reinterpret_cast<uint8_t*>(mKeyVals + numElementsWithBuffer);
        std::memset(mInfo, 0, numBytesTotal - (numElementsWithBuffer * sizeof(Node)));
 
        // set sentinel
        mInfo[numElementsWithBuffer] = 1;
 
        mInfoInc = InitialInfoInc;
        mInfoHashShift = InitialInfoHashShift;
      }
 
      enum class InsertionState { overflow_error, key_found, new_node, overwrite_node };
 
      // Finds key, and if not already present prepares a spot where to pot the key & value.
      // This potentially shifts nodes out of the way, updates mInfo and number of inserted
      // elements, so the only operation left to do is create/assign a new node at that spot.
      template <typename OtherKey>
      std::pair<size_t, InsertionState> insertKeyPrepareEmptySpot(OtherKey&& key) {
        for (int i = 0; i < 256; ++i) {
          size_t idx{};
          InfoType info{};
          keyToIdx(key, &idx, &info);
          nextWhileLess(&info, &idx);
 
          // while we potentially have a match
          while (info == mInfo[idx]) {
            if (WKeyEqual::operator()(key, mKeyVals[idx].getFirst())) {
              // key already exists, do NOT insert.
              // see http://en.cppreference.com/w/cpp/container/unordered_map/insert
              return std::make_pair(idx, InsertionState::key_found);
            }
            next(&info, &idx);
          }
 
          // unlikely that this evaluates to true
          if GAIA_UNLIKELY (mNumElements >= mMaxNumElementsAllowed) {
            if (!increase_size()) {
              return std::make_pair(size_t(0), InsertionState::overflow_error);
            }
            continue;
          }
 
          // key not found, so we are now exactly where we want to insert it.
          auto const insertion_idx = idx;
          auto const insertion_info = info;
          if GAIA_UNLIKELY (insertion_info + mInfoInc > 0xFF) {
            mMaxNumElementsAllowed = 0;
          }
 
          // find an empty spot
          while (0 != mInfo[idx]) {
            next(&info, &idx);
          }
 
          if (idx != insertion_idx) {
            shiftUp(idx, insertion_idx);
          }
          // put at empty spot
          mInfo[insertion_idx] = static_cast<uint8_t>(insertion_info);
          ++mNumElements;
          return std::make_pair(
              insertion_idx, idx == insertion_idx ? InsertionState::new_node : InsertionState::overwrite_node);
        }
 
        // enough attempts failed, so finally give up.
        return std::make_pair(size_t(0), InsertionState::overflow_error);
      }
 
      bool try_increase_info() {
        ROBIN_HOOD_LOG(
            "mInfoInc %zu, numElements=%zu, maxNumElementsAllowed=%zu", mInfoInc, mNumElements,
            calcMaxNumElementsAllowed(mMask + 1));
        if (mInfoInc <= 2) {
          // need to be > 2 so that shift works (otherwise undefined behavior!)
          return false;
        }
        // we got space left, try to make info smaller
        mInfoInc = static_cast<uint8_t>(mInfoInc >> 1U);
 
        // remove one bit of the hash, leaving more space for the distance info.
        // This is extremely fast because we can operate on 8 bytes at once.
        ++mInfoHashShift;
        auto const numElementsWithBuffer = calcNumElementsWithBuffer(mMask + 1);
 
        for (size_t i = 0; i < numElementsWithBuffer; i += 8) {
          auto val = unaligned_load<uint64_t>(mInfo + i);
          val = (val >> 1U) & UINT64_C(0x7f7f7f7f7f7f7f7f);
          memcpy(mInfo + i, &val, sizeof(val));
        }
        // update sentinel, which might have been cleared out!
        mInfo[numElementsWithBuffer] = 1;
 
        mMaxNumElementsAllowed = calcMaxNumElementsAllowed(mMask + 1);
        return true;
      }
 
      // True if resize was possible, false otherwise
      bool increase_size() {
        // nothing allocated yet? just allocate InitialNumElements
        if (0 == mMask) {
          initData(InitialNumElements);
          return true;
        }
 
        auto const maxNumElementsAllowed = calcMaxNumElementsAllowed(mMask + 1);
        if (mNumElements < maxNumElementsAllowed && try_increase_info()) {
          return true;
        }
 
        ROBIN_HOOD_LOG(
            "mNumElements=%zu, maxNumElementsAllowed=%zu, load=%f", mNumElements, maxNumElementsAllowed,
            (static_cast<double>(mNumElements) * 100.0 / (static_cast<double>(mMask) + 1)))
 
        if (mNumElements * 2 < calcMaxNumElementsAllowed(mMask + 1)) {
          // we have to resize, even though there would still be plenty of space left!
          // Try to rehash instead. Delete freed memory so we don't steadyily increase mem in case
          // we have to rehash a few times
          nextHashMultiplier();
          rehashPowerOfTwo(mMask + 1, true);
        } else {
          // we've reached the capacity of the map, so the hash seems to work nice. Keep using it.
          rehashPowerOfTwo((mMask + 1) * 2, false);
        }
        return true;
      }
 
      void nextHashMultiplier() {
        // adding an *even* number, so that the multiplier will always stay odd. This is necessary
        // so that the hash stays a mixing function (and thus doesn't have any information loss).
        mHashMultiplier += UINT64_C(0xc4ceb9fe1a85ec54);
      }
 
      void destroy() {
        if (0 == mMask) {
          // don't deallocate!
          return;
        }
 
        Destroyer<Self, IsFlat && std::is_trivially_destructible<Node>::value>{}.nodesDoNotDeallocate(*this);
 
        // This protection against not deleting mMask shouldn't be needed as it's sufficiently
        // protected with the 0==mMask check, but I have this anyways because g++ 7 otherwise
        // reports a compile error: attempt to free a non-heap object 'fm'
        // [-Werror=free-nonheap-object]
        if (mKeyVals != reinterpret_cast_no_cast_align_warning<Node*>(&mMask)) {
          ROBIN_HOOD_LOG("gaia::mem::mem_free");
          gaia::mem::mem_free(mKeyVals);
        }
      }
 
      void init() noexcept {
        mKeyVals = reinterpret_cast_no_cast_align_warning<Node*>(&mMask);
        mInfo = reinterpret_cast<uint8_t*>(&mMask);
        mNumElements = 0;
        mMask = 0;
        mMaxNumElementsAllowed = 0;
        mInfoInc = InitialInfoInc;
        mInfoHashShift = InitialInfoHashShift;
      }
 
      // members are sorted so no padding occurs
      uint64_t mHashMultiplier = UINT64_C(0xc4ceb9fe1a85ec53); // 8 byte  8
      Node* mKeyVals = reinterpret_cast_no_cast_align_warning<Node*>(&mMask); // 8 byte 16
      uint8_t* mInfo = reinterpret_cast<uint8_t*>(&mMask); // 8 byte 24
      size_t mNumElements = 0; // 8 byte 32
      size_t mMask = 0; // 8 byte 40
      size_t mMaxNumElementsAllowed = 0; // 8 byte 48
      InfoType mInfoInc = InitialInfoInc; // 4 byte 52
      InfoType mInfoHashShift = InitialInfoHashShift; // 4 byte 56
                                                      // 16 byte 56 if NodeAllocator
    };
 
  } // namespace detail
 
  // map
 
  template <
      typename Key, typename T, typename Hash = hash<Key>, typename KeyEqual = gaia::core::equal_to<Key>,
      size_t MaxLoadFactor100 = 80>
  using unordered_flat_map = detail::Table<true, MaxLoadFactor100, Key, T, Hash, KeyEqual>;
 
  template <
      typename Key, typename T, typename Hash = hash<Key>, typename KeyEqual = gaia::core::equal_to<Key>,
      size_t MaxLoadFactor100 = 80>
  using unordered_node_map = detail::Table<false, MaxLoadFactor100, Key, T, Hash, KeyEqual>;
 
  template <
      typename Key, typename T, typename Hash = hash<Key>, typename KeyEqual = gaia::core::equal_to<Key>,
      size_t MaxLoadFactor100 = 80>
  using unordered_map = detail::Table<
      sizeof(robin_hood::pair<Key, T>) <= sizeof(size_t) * 6 &&
          std::is_nothrow_move_constructible<robin_hood::pair<Key, T>>::value &&
          std::is_nothrow_move_assignable<robin_hood::pair<Key, T>>::value,
      MaxLoadFactor100, Key, T, Hash, KeyEqual>;
 
  // set
 
  template <
      typename Key, typename Hash = hash<Key>, typename KeyEqual = gaia::core::equal_to<Key>,
      size_t MaxLoadFactor100 = 80>
  using unordered_flat_set = detail::Table<true, MaxLoadFactor100, Key, void, Hash, KeyEqual>;
 
  template <
      typename Key, typename Hash = hash<Key>, typename KeyEqual = gaia::core::equal_to<Key>,
      size_t MaxLoadFactor100 = 80>
  using unordered_node_set = detail::Table<false, MaxLoadFactor100, Key, void, Hash, KeyEqual>;
 
  template <
      typename Key, typename Hash = hash<Key>, typename KeyEqual = gaia::core::equal_to<Key>,
      size_t MaxLoadFactor100 = 80>
  using unordered_set = detail::Table<
      sizeof(Key) <= sizeof(size_t) * 6 && std::is_nothrow_move_constructible<Key>::value &&
          std::is_nothrow_move_assignable<Key>::value,
      MaxLoadFactor100, Key, void, Hash, KeyEqual>;
 
} // namespace robin_hood
 
#endif